1. Field of the Invention
The present invention is directed to the field of devices for restraining and selectively releasing a tension force. More particularly, the present invention is directed to the field of devices that deploy folded structures on a spacecraft after launch.
2. Background Art
It is common in the space industry to launch a spacecraft with some of its parts folded down against the exterior of the spacecraft. For example, when the spacecraft is a solar powered earth satellite, the large solar panels must be folded down so that the spacecraft can fit inside the payload space of a launch vehicle. Antennae, sensors, and nuclear power plants (mounted on extensible members) are examples of other bulky appendages that may need to be folded down to the side of a spacecraft during launch.
After the spacecraft has been placed in space, the folded parts must then be reliably deployed. Typically, this is done via a spring bias that tends to urge the folded part into an extended position. During launch, a retention member, which will later be released upon a remote command, counters the spring bias. The retention member is commonly referred to in the art as the "pin," although it sometimes is not literally in the form of a pin. The tension release device that releases the retention member (or pin) is commonly known in the art as the "pin puller."
Pyrotechnic devices have been long used as tension release devices. Commonly known as exploding nuts or exploding bolts, pyrotechnic devices are a mature technology that has the advantages of providing for very secure retention of folded structures and deploying reliably, that is, they are mechanically strong and have a low failure rate.
One disadvantage of pyrotechnics is that they are susceptible to premature firing. Static electricity and electromagnetic radiation can accidentally set them off. Premature firing causes untimely deployment of folded parts and other potentially catastrophic results. Thus, these prior art tension release devices are risky due to their tendency to prematurely actuate as a result of ambient conditions.
Another disadvantage common to the prior art tension release (i.e., pin puller) schemes is that they are high shock devices. Pyrotechnic devices, by their very nature, explode. The explosion sends mechanical shock waves all through the spacecraft. In addition, they impart subtle forces on the orbital mechanics of the vehicle. These shock waves place additional stress on the spacecraft that can only increase the risk of failure of delicate systems aboard the spacecraft, such as electronic circuits, electrical connectors, and optical sensors.
Another approach to tension release devices has been paraffin actuators. Upon receiving a remote command, the paraffin actuator develops a linear mechanical force to actuate a release mechanism, thereby deploying the folded parts of the spacecraft. The paraffin actuator operates based on the phenomenon that when paraffin changes from solid phase to liquid phase it undergoes a substantial expansion. A cylindrical slug of solid paraffin is loaded in a cylinder with a piston disposed against it. When a remote deployment command is received, heat is applied to the cylinder containing the paraffin, causing the paraffin to undergo a phase change. The resulting expansion of the paraffin forces the piston to move through the cylinder and develops a mechanical force. Pin pullers based on paraffin actuators also generate shock waves due to the suddenness with which the restraining force is released.
A failed attempt at improving on the above devices involved the use of a low temperature melting point metal alloy as a fusible link in the tension release device. This device was initially described by William D. Nygren, "Development of a High Force Thermal Latch" 29th Aerospace Mech. Symposium, May 17-19, 1995. This device is also described in U.S. Pat. No. 5,695,306. Upon receiving a remote command for deployment, heat was applied to the fusible link 100, 300 so that it would melt, thereby causing the tension release device to undergo a mechanical shift so as to permit release of the pin. This system failed because the fusible link 100, 300 was directly bearing more force than it could handle. The fusible link 100, 300 was unable to withstand the mechanical forces exerted on it and, thus, the fusible link 100, 300 tended to flow prematurely due to tension forces alone. Accordingly, this prior art tension release device is not commercially viable. The force that a tension release device must restrain is on the order of several thousand pounds, typically 5,000 to 10,000 pounds, for a folded solar panel array.
Another prior art tension release device is disclosed in published EPO patent application no. 0 441 669. This tension release device restrains a pin 8 that is holding down folded solar panels 3A, 3B on a spacecraft 4. The pin 8 is held in place by balls 12a, 12b, 12c, which are in turn held in place by a circular cam 16. When pyrotechnic devices 23 ignite, the restraint pin 22 is removed so that the cam 16 is free to rotate and thereby move the balls 12a, 12b, 12c downward and release the pin 8. By its use of pyrotechnic devices, it is clear that this invention does not contemplate solving either the problem of premature actuation or the problem of generation of large shock waves.
Thus, what is needed is a tension release device (i.e., pin puller) that operates reliably, that avoids premature actuation, and that avoids generation of large shock waves due to sudden actuation.